Crystalline thermoplastic polybutylene terephthalate (PBT) resins have been used extensively as an engineering plastic in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics. Thus, thermoplastic PBT resins have been used to form molded parts in various fields including automotive, electrical and electronic end-use applications.
The injection molding of PBT is known to be generally easier as compared to the injection molding of polyethylene terephthalate (PET) resins. However, when PBT resins are molded into relatively small articles or articles having a complex shape and/or thin walls, various problems ensue such as distortion and breakage of the articles owing to poor mold release properties. As a result, there have been previous attempts to improve the processability of PBT during injection molding by the addition of various mold-release agents, such as paraffin oil, fatty acid amides and fatty acid esters.
However, PBT injection-molding compositions which contain a conventional mold-release agent exhibit problems during the injection-molding process. That is, PBT compositions which contain conventional mold-release agents typically exhibit poor "bite" in the extruder which results in the pellets slipping on the screw of the injection-molding machine. This "slippage" of conventional PBT resins is presumably due to the bleed-through of the mold-release agents when the pellets are pre-dried prior to injection-molding. As a result, the pellets are insufficiently plasticized and therefore are injection-molded with some difficulty. Furthermore, when injection-molded articles are formed, they are often unstable (e.g. degrade and/or discolor) over prolonged periods of use.
It is also conventional and highly desirable to recycle scrap thermoplastic material in order to reduce the production costs of final articles. However, the problem of poor extruder screw "bite" becomes even more apparent when scrap thermoplastic material is employed in the injection-molding process. That is, when scrap thermoplastic material is added to virgin thermoplastic material, poor extruder screw "bite" will cause insufficient mixing and plasticization of the scrap and virgin thermoplastic material.
The problem of poor extruder screw "bite" has been solved to some extent by the deposition of a metal soap on the surface of the pellets prior to injection-molding. However, deposition of a metal soap presents its own problems in that it further complicates the injection-molding operation (e.g. since a further additive must be handled and processed), as well as contributing to corrosion of metal parts of the processing machinery in which it comes into contact.
What has been needed, therefore, is a polybutylene terephthalate injection-molding resin composition which is relatively easily processable (i.e., exhibits sufficient extruder screw "bite" during injection molding), while at the same time has satisfactory mold-release properties and long-term stability. It is towards attaining such a PBT injection-molding resin that the present invention is directed.
Broadly, the present invention is embodied in stable, injection-moldable PBT resin compositions which not only exhibit excellent mold-release characteristics, but which also have satisfactory extruder screw "bite" when pellets of the composition are employed in injection-molding machines. As a result, the PBT injection-molding compositions may be employed to form injection molded articles which remain stable after prolonged periods of use.
In specific embodiments, the PBT injection-molding compositions of the present invention include (A) 100 parts by weight of a polybutylene terephthalate base resin melt-blended with (B) between about 0.01 to 10 parts by weight of a processing aid which is a sorbitan ester of a fatty acid having at least 12 carbon atoms.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.